Apparatus for reproducing an optical recording medium having first and second pit strings on opposite sides of each track

ABSTRACT

There is provided an optical recording medium including a first pit string having a succession of pits and mirror surface sections, which first pit string is formed on one side of a track center as a reference, and a second pit string having pits and mirror sections, reversed in their array from the pits and the mirror surface sections of the first pit string. A laser beam is radiated onto the track center for accessing information signals represented by the pits and the mirror surface sections. There are also provided a recording device and a reproducing device for such optical recording medium.

This is a divisional of application Ser. No. 08/280,181, filed Jul. 25,1994 now U.S. Pat. No. 5,563,872.

BACKGROUND OF THE INVENTION

This invention relates to an optical recording medium in which a dataarea is constituted by a pit string made up of pits formed along thetrack center scanned by a laser beam and lands (mirror surface sections)and in which the pit string in the data area is read with pre-set clocksso as to be reproduced as information signals, a method for reproducingdata recorded on the optical recording medium, and a data recordingdevice or laser cutting device employed for producing the opticalrecording medium.

An optical recording medium, such as one rotated at CAV (constantangular velocity), referred to herein as an optical disc, has arecording format, such as a recording format for a data area as shown inFIG. 21, in which pits P are formed along a track center Tc, with a pitwidth d of 0.5 μm and a pit length per clock of 0.86 μm and at a trackpitch Tp of 1.6 μm along the disc radius, and are aligned in theirpositions along the track direction. The portions devoid of the pits arelands left as mirror surface regions.

For reproducing information signals from the optical disc having theabove-mentioned recording format, the disc is rotated at CAV and aplayback laser beam is radiated on the track center Tc for relativescanning with a laser spot BS.

The laser beam reflected from the optical disc is caused to fall on alight receiving element and converted into detected signals aselectrical signals having a signal level corresponding to the reflectedlight volume. The detected signals are further demodulated by a signaldemodulating circuit for producing playback information data.

The reflected light falling on the light receiving element afterreflection by the mirror surface region is the light which has undergonesubstantially total reflection by the mirror surface. Thus the reflectedlight volume is abundant so that a detection signal having a high signallevel is outputted from the light receiving element. On the other hand,the light volume of the reflected light modulated by the pit is lesserthan that of the reflected light from the mirror surface region, so thata detection signal having a low signal level is outputted from the lightreceiving element.

In a downstream signal processor, detection signals outputted in seriesby the light receiving element are sampled with pre-set clock signalsand thereby converted to bi-level data having a pulse amplitudecorresponding to the signal level. The bi-level data is processed fordecoding error-correction codes, such as parity codes or interleaving,for producing playback information data.

Since data corresponding to the pit P is a logical "1" and datacorresponding to the mirror surface region is a logical "0", data havinga long concatenation of "1"s or "0"s suffers from increased deviation inthe dc balance. That is, the digital sum value (DSV) is offset to the(-) side or to the (+) side, thus producing an unstable state of theservo control system.

In addition, such data recording has a drawback that the data length issubstantially increased, which is not meritorious in increasing the datarecording density.

Another known recording method is shown in FIG. 22 in which recording ismade so that the boundary between the pit P and the mirror surfaceregion M is logically "1" and the pit P as well as the mirror surfaceregion M other than the boundary is logically "0". Data reproduction ismade in a similar manner, Such recording method is meritorious forincreasing data recording density since it is unnecessary to increasethe data length in distinction from the firstly stated system.

Consequently, the conventional practice has been to record pits on theoptical disc after 8-bit data is converted to 14-bit data in accordancewith the eight-to-fourteen modulation (EFM). Playback information dataare produced after decoding EFM codes in addition to decoding of theerror correction codes, as stated hereinabove.

However, the EFM system is not meritorious for high-density recordingbecause it is 14-bit data converted from 8-bit data that is to berecorded. Although it is desirable to employ a direct data recordingsystem for high density recording, the above-mentioned problem caused byincreased dc balance offset is raised.

Besides, with the conventional optical disc, since the recording dataare implemented by a bit string pattern consisting of mirror surfacesections M and pits P formed on the track center Tc, the number of pitsP and a range in which the pits P are formed differ from track to trackdepending on recording data. That is, the proportion between the numberor size of the pits P and the number or size of the lands M in a dataarea per track is not equal and differs from track to track.

This leads to such a situation in the optical disc fabrication that,when a recording pattern on a stamper (a pit array pattern consisting ofpits P and lands M) is to be transcribed onto a resin substrate by aninjection molding method using the stamper, the flow rate of the moltenresin into cavities is not uniform due to the differential density ofthe protrusions and recesses on the stamper resulting in fluctuations inthe state of adhesion of the molten resin to the stamper. The result isthat the contour of the pits P of the completed optical disc is locallydeviated from the prescribed shape, while molding defects such asinterruptions in the mirror surface sections M are produced.

Such molding defects are produced in particular in servo areas of theoptical disc constructed in accordance with the sampled servo system.That is, the servo area is usually separated from the data area by amirror area constituted solely by mirror surface sections M. Thus themirror area is continuous along the radius of the optical disc. Theresult is that molten resin flows quickly through the mirror areatowards the outer periphery of the cavity, so that a so-called ghost,that is broken edges of servo pits caused by the radially continuousmirror area, tends to be produced.

In addition, with the above-described sampled servo type optical disc,the following problem arises during tracking servo control duringreproduction.

That is, the tracking servo control in the conventional sampled servosystem is carried out using a pair of wobbled pits Pa and Pb pre-formedwith a shift of one-fourth of a track pitch in opposite directions fromthe track center Tc, as shown in FIG. 23.

Specifically, the amount of reflected light when the laser beam spot BStraverses the wobbled pits Pa and Pb is sampled, and a tracking errorsignal is found based upon the difference between these signals. Thespot BS is moved radially of the optical disc until the signal level,for example, of the tracking error signal becomes equal to zero, by wayof performing tracking servo control.

On the other hand, the so-called track jump, which is the movement ofthe spot to a neighboring or other track, is performed by opening thetracking servo control loop, moving the spot to near a target track andsubsequently closing the tracking servo control loop for capturing thespot BS to the target track.

During such track jump, that is when the laser beam spot BS scans thetrack obliquely, the tracking error signal in the sampled servo systemis a sine wave signal, as shown in FIG. 24, such that it is notunequivocally set with respect to a displacement x of the beam spot BSfrom the track center.

It is when the spot BS is within a range 201, shown by hatched lines,that the tracking error signal is determined unequivocally with respectto the displacement x. That is, it is when the spot BS is within therange 201 that the spot BS can be captured without fail with respect tothe track center Tc.

On the other hand, if the displacement x is larger and is outside therange 201, that is within the range 202, tracking servo control becomesunstable. Such unstable state tends to be produced when the laser beamspot BS is moved with an elevated speed along the radius of the opticaldisc as during the track jump.

There is produced an error in the distance the beam spot BS is movedduring track jump with the tracking servo loop being turned off. If thetracking servo control loop is turned on outside the range 201, such aswithin the range 202, the possibility is high that the beam spot will becaptured to a track other than the target track. In such case, a trackjump needs to be performed a second time. Thus the conventional trackingservo has a drawback that the track jump cannot be preformed stably.

SUMMARY OF THE INVENTION

In view of the above-described status of the art, it is an object of thepresent invention to provide an optical recording medium in which, evenif data includes a succession of continuous logical "1"s or "0"s, dcbalance may be optimized, that is the DSV may be drawn closer to zero,without the necessity of performing modulation, such as EFM, which mightotherwise produce increased data lengths, and in which stabilization inservo control and high density in the recording data may be achievedsimultaneously.

It is another object of the present invention to provide an opticalrecording medium in which the proportion of the size of pits and that ofthe lands in the data area per track may be rendered equal to eachother, so that, when transcribing a recording pattern (pit stringpattern consisting of pits and lands) on the stamper onto the resinsubstrate by, for example, resin injection molding, the flow velocity ofthe molten resin into the stamper cavities may be rendered uniform forthe cavities in their entirety, thereby eliminating the molding defectsduring fabrication of the optical recording media. It is a furtherobject of the present invention to provide an optical recording mediumin which the servo area may be detected easily when the opticalrecording medium is of the sampled servo type.

According to the present invention, there is provided an opticalrecording medium comprising a first pit string having a succession ofpits and mirror surface sections and being formed on one side of a trackcenter as a reference, and a second pit string having pits and mirrorsections, logically inverted in their array from the pits and the mirrorsurface sections of the first pit string. The laser beam is radiatedonto the track center for accessing information signals represented bythe pits and the mirror surface sections.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a), 1(b) and 1(c) are dramatic views illustrating a recordingformat of a sampled servo read-only optical recording disc according toa first embodiment of the present invention and particularlyillustrating a data segment, an address segment and an address/datasegment, respectively.

FIGS. 2(a), 2(b) and 2(c) are dramatic views for illustrating the typesof sectors of the optical disc of the first embodiment, and inparticular show a 512-byte sector, a 1024-byte sector and a 2048-bytesector, respectively.

FIG. 3 schematically shows an example of a recording format in a servoarea and a near-by data area in the optical disc of the firstembodiment.

FIGS. 4(a), 4(b), 4c) and 4(d) show essential parts of the optical discof the first embodiment, especially showing its servo region and itsvicinity, wherein FIG. 4(a) schematically shows an example of arecording format for the servo area and its vicinity, FIG. 4(b) shows apush-pull signal waveform produced on reproducing the servo area and itsvicinity, FIG. 4(c) shows an RF signal waveform produced on reproducingthe servo area and its vicinity, and FIG. 4(d) shows waveforms of amirror region detecting gating signal, a data area detecting gatingsignal, a servo area detecting gating signal, a clock detecting gatingsignal, an address data region detecting gating signal and clock pulses.

FIGS. 5(a), 5(b) and 5(c) shows essential portions, above all, data, ofthe optical disc according to the first embodiment, and in particularshow an example of the recording format in the recording data section ofthe data region, a push-pull signal waveform produced on reproducing thedata region and an RF signal waveform produced on reproducing the dataregion, respectively.

FIG. 6 is a diagrammatic view illustrating bit allocation to addressdata in an address data section in an address segment in the opticaldisc according to the first embodiment.

FIG. 7 is a schematic view showing the contents of the gray coderepresented by pits formed in an address data section in an addresssegment in the optical disc according to the first embodiment.

FIG. 8 is a block diagram showing an arrangement of a playback system ofa disc reproducing device according to an embodiment of the presentinvention.

FIG. 9 shows an arrangement of an optical pickup in the playback systemof a disc reproducing device according to an embodiment of the presentinvention.

FIGS. 10(a) to (d) show the intensity distribution of the reflectedlight on radiation of a laser beam to the optical disc according to thefirst embodiment.

FIG. 11 is a diagrammatic view showing the plan configuration of alight-receiving surface of a photodetector in the optical pickup shownin FIG. 9.

FIGS. 12(a) through 12(d) show a far-field pattern of the reflectedlight on the light-receiving surface of the photodetector.

FIGS. 13(a) through 13(e) are timing charts showing the operation of thetracking servo in the playback system in the disc reproducing deviceaccording to the embodiment shown in FIG. 8.

FIG. 14 is a circuit diagram of an example of a tracking error signalgenerating circuit built into the reproducing system, above all, theservo error signal generating circuit, according to the embodiment shownin FIG. 8.

FIG. 15 is a circuit diagram showing another embodiment of the trackingerror signal generating circuit.

FIG. 16 is a timing chart showing the operation of the tracking servo,above all, track jump, in the reproducing system of the disc reproducingdevice according to the embodiment shown in FIG. 8.

FIG. 17 is a block diagram showing an arrangement of a laser cuttingdevice employed for producing the optical disc according to the firstembodiment.

FIG. 18 is a schematic view showing essential portions of an opticaldisc according to a second embodiment, above all, another embodiment ofa recording format in its servo region and its vicinity.

FIG. 19 is a schematic view showing essential portions of an opticaldisc according to a third embodiment, above all, still anotherembodiment of a recording format in its servo region and its vicinity.

FIG. 20 is a schematic view showing essential portions of an opticaldisc according to a fourth embodiment, above all, a further embodimentof a recording format in its servo region and its vicinity.

FIG. 21 is a schematic view showing a recording format of a conventionaloptical disc.

FIG. 22 is a schematic view showing the playback logic for theconventional optical disc.

FIG. 23 is a schematic view showing the format for the servo pits of theconventional optical disc.

FIG. 24 is a waveform diagram showing tracking error signals produced onreproducing the conventional optical disc.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 to 20, preferred illustrative embodiments of theoptical recording medium, as applied to a read-only optical disc of thesampled servo system, referred to herein as an optical disc, areexplained in detail.

The optical disc is of the type run in rotated at a constant angularvelocity (CAV) and has a recording format in which each track is dividedinto plural sectors each of which is made up of plural segments.Referring to FIGS. 1(a) to 1(c), each segment is made up of a 3-byteservo region Zs having servo pits and a 34-byte data region Zd forrecording data per se. By the format constitution of the data region Zd,the segments are classified into three types of segments.

The first type segment is a data segment constituted by the three-byteservo region Zs followed by a recording data region 2d made up of a1-byte clamp area Zc and a 33-byte recording data region Zw, as shown inFIG. 1(a). The second type segment is an address segment constituted bythe three-byte servo region Zs followed by a data region Zd made up of a1-byte clamp area Zc, a 14-byte address data area Za, a 2-byte automaticpower control (APC) data and a 17-byte blank Zb, as shown in FIG. 1b.The third type segment is an address/data segment constituted by thethree-byte servo region Zs followed by a data region Zd made up of a1-byte clamp area Zc, a 14-byte address data area Za, a 2-byte APC dataand a 17-byte data region Zw, as shown in FIG. 1c.

There are also three types of sectors, namely a 512-byte sector, a1024-byte sector and a 2048-byte sector.

The 512-byte sector comprises a series array of a first segment stringmade up of the leading address/data segment followed by nine datasegments and a second segment string made up of the leading addresssegment followed by the nine data segments, as shown in FIG. 2(a). Theactual recording capacity of the recording data is 611 bytes, as shownby the following formula (1):

    1 sector=9×2 segments×33 bytes+17×1 bytes=611 bytes(1)

The 1024-byte sector comprises a series array of two of theabove-mentioned first segment strings and two of the above-mentionedsecond segment strings, arranged in alternation with one another, asshown in FIG. 2(b). The actual recording capacity of the recording datais 1222 bytes, as shown by the following formula (2):

    1 sector=9×4 segments×33 bytes+17×2 bytes=1222 bytes(2)

The 2048-byte sector comprises a series array of four of theabove-mentioned first segment strings and four of the above-mentionedsecond segment strings, arranged in alternation with one another, asshown in FIG. 2(c). The actual recording capacity of the recording datais 2444 bytes, as shown by the following formula (3):

    1 sector=9×8 segments×33 bytes+17×4 bytes=2444 bytes(3)

A first embodiment of the optical disc having the above-mentioned formatconstitution, above all, its recording format, is explained by referringto FIGS. 3 to 5. The recording format of the optical disc according tothe first embodiment has the servo region Zs made up of a mirror area Zmconstituted solely by a mirror surface and a servo area Zss constitutedby an array of servo pits, with the servo area Zss being separated bythe mirror area Zm from the data region Zd, as shown in FIGS. 3 and 4.That is, the servo region Zs is made up of the servo area Zss at acenter and two mirror areas Zm on either sides of the servo area Zss.

The recording format of the data region Zd, above all, the recordingdata area Zw, of the optical disc, is such that a pit string made up ofpits P and lands (mirror surface sections) M formed on the radiallyinner peripheral side of the track center Tc, and an inverted pit stringmade up of the pits P and the lands M formed on the radially outerperipheral side of the track center Tc, with the second sequence of thepits and the lands being the inverse of the firstly mentioned pitstring, as shown in FIG. 5(a). The effect is such that whereever thereis a P on the radially inner peripheral side of the track center T_(c),there is a land M on the radially outer peripheral side of the trackcenter T_(c), and vice versa. One-track data is constituted by the pitstring and the inverted pit string, arranged on either sides of thetrack center.

Specifically, if the diameter of the reproducing laser beam on therecording surface is set to 1.5 to 1.6 μm, similarly to that used for aconventional compact disc reproducing device, a number of pits P, eachhaving a pit width d of 0.5 μm, are formed on the radially inner andouter peripheral sides of the track center Tc, as in the case of theconventional compact disc.

The distance between the track centers Tc, that is the track pitch, isselected to be 1.6 μm and, as shown in FIG. 4(a), the distance t_(a)between the inner side pit string in the data region Zd of a giventrack, such as the first track T₁, and the outer side inverted pitstring in the data region Zd of the neighboring track T_(a), is selectedto be equal to the distance t_(b) between the inner side pit string andthe outer side inverted pit string for the track T₁.

That is, with the optical disc of the present first embodiment, theinner side pit string is formed at a distance corresponding toone-fourth of a track pitch on the inner side of the track center Tc,while the outer side pit string is formed at a distance corresponding toone-fourth of the track pitch on the outer side of the track center Tc.That is, pit strings are arranged along the track direction at a pitchequal to one-half of the track pitch along the radius of the opticaldisc.

The spot BS of the playback laser beam scans the track center Tc, and isof such a size that both the inner side pits P and the outer side pits Pare encompassed within the beam spot BS.

The relation of pit strings on the inner and outer sides of the trackcenter Tc, as seen from the scanning spot BS, is such that, if there isthe pit P on the inner side, it is faced by the land M on the outerside, whereas, if there is the land M on the inner side, it is faced bythe pit P on the outer side.

Consequently, as long as the recording data area Zw of the data regionZd of the disc is scanned by the spot BS, there necessarily exists oneof the pits within the beam spot BS.

For reproducing the optical disc of the present first embodiment, clocksignals are generated during reproduction from a clock mark Mc formed inthe servo area Zss of the servo region Zs, as later explained, and thepit string and the inverted pit string of the data region Zd arereproduced based upon output timings of the generated clock signals. InFIGS. 4(a) and 5(a), vertical ruled lines schematically indicate theoutput timings of the clock signals.

It is thus seen that the leading and trailing ends of the pits P in boththe pit string and the inverted pit string in the recording data area Zware formed in synchronism with the output timings of the clock signals,as shown in FIG. 5(a). On the other hand, the logical data reproducedfrom the pit string has a constitution in which the boundary between thepit P and the land M, that is the above-mentioned leading or trailingend, is logically "1", while the portions of the pit P and the lands Mother than the boundary are logically "0".

The servo area Zss in the servo region Zs is sandwiched between mirrorareas (lands) Zm formed on either sides of the track, as shown in FIG.4(a). The leading end of the servo area Zss is arranged at a positiondisplaced 1.5 clocks from the trailing end of the data region Zd, whilethe trailing end of the servo area Zss is arranged at a positiondisplaced 3.5 clocks from the leading end of the data region Zd.

That is, the data region Zd and the servo area Zss are spaced apart fromeach other by the mirror area corresponding to 1.5 clocks, that is aninteger and a fractional number of clocks, instead of an integer numberof clocks, thereby enabling the distinction to be made between the dataregion Zd and the servo region Zs.

Besides, the same arraying pattern of the pits (servo pits) Pconstituting the servo area Zss is repeated at an interval of threetracks. Specifically, if the servo area Zss is divided at an interval oftwo clocks into three zones, beginning from the leading end, with thefirst, second and third zones being termed regions A, B and C, as shownin FIG. 4(a), the combinations of pit arrays contained in the threezones are of three different patterns.

In the illustrated example, pits P are formed in the zones A and C onthe inner side of the first track T₁, while a sole pit P is formed onthe outer side thereof across the zones A and B. That is, the pits P areformed on both the inner and outer sides of the zone A, whereas the pitsP are formed on the outer side of the zone B and on the inner side ofthe zone C. With the first track T₁, the pits in the zone A are used asclock marks Mc for clock detection, while the pits P in the zones B andC are used as a servo mark Ms for tracking error detection.

On the other hand, a pit P is formed across the zones B and C on theinner side of the second track T₂, while pits P are formed on the outerside thereof in the zones A and C. That is, the pits P are formed onboth the inner and outer sides of the zone C, whereas the pits P areformed on the outer side of the zone A and on the inner side of the zoneB. With the second track T₂, the pits in the zone C are used as clockmarks Mc for clock detection, while the pits P in the zones A and B areused as a servo mark Ms for tracking error detection.

On the other hand, a pit P is formed across the zones A and B on theinner side of the third track T₃, while a sole pit P is formed on theouter side thereof in the zones A and C. That is, the pits P are formedacross the inner side and the outerside of zone B, whereas the pits Pare formed on the inner side of the zone A and on the outer side of thezone C. With the third track T₃, the pits P in the zone B are used asclock marks Mc for clock detection, while the pits P in the zones A andC are used as a servo mark Ms for tracking error detection.

The method for tracking servo control by the above-described pit arrayof the servo region Zs will be explained in detail subsequently.

The recording format of the data region Zd, especially that of theaddress segment or the address/data segment, of the optical disc of thefirst embodiment, comprises a recorded series array of an access codeand a sector code, each representing the Gray code by a pit string andan inverted pit string, as shown in FIGS. 3 and 6. In FIG. 3, only theaccess code is shown.

Taking an example of the address segment, the access code is made up ofan upper order address area (AH, AM_(H)) indicating an upper orderaddress of the track address and a lower order address area (AM_(L), AL)indicating the lower order address thereof, with the upper address areabeing divided into two address areas AH and AM_(H) and with the loweraddress area being divided into two address areas AM_(L) and AL, asshown in FIG. 6. Each of the address areas AH, AM_(H), AM_(L) and AL isformed by four bits and allocated to a 14-clock area based upon outputtimings of the clock signals. With each of the address areas AH, AM_(H),AM_(L) and AL, the Gray code is represented by the pit array formed atthe central 12-clock area, as shown in FIG. 7.

A sector code is made up of an upper address area S_(H) and a loweraddress area S_(L), indicating the upper order and lower order addressesof the sector address, and inverted address areas S_(L) and S_(H)inverted from these data. Similarly to the access code, the sector codehas an address area arranged to represent the Gray code by the pit arrayformed in a central 12-clock area.

Turning to the pit string representing the Gray code, the innerperipheral side pit string is constituted by a leading pit Pt and atrailing end pit Pr, each having a pit length corresponding to therelevant Gray code, and a land M having a length corresponding to therelevant Gray code, while the outer peripheral side inverted pit stringis constituted by a leading end land M1 and a trailing end land M2, eachhaving a pit length corresponding to the relevant Gray code, and acentral pit P having a length corresponding to the relevant Gray code,as shown in FIG. 7.

The leading end pit Pt and the trailing end pit Pr in the innerperipheral side pit string are overlapped by two clocks with the centralpit Pc in the outer peripheral side inverted pit string, such that theGray code recorded in each address areas AH, AM_(H), AM_(L), AL, S_(H)and S_(L) may be read by detecting the overlapped portions by theplayback laser beam.

In the above pit string, it is the pit string portion and the invertedpit string portion corresponding to the central 12 clocks that are readas the Gray code. The 1-clock pits on either ends play the part ofnegative polarity marks used for rendering the push-pull signal intonegative polarity signal.

The clamp area Zc comprises a staggered array of clamp pits P_(CLMP),each having a pit length corresponding to one clock, as shown in FIGS. 3and 4(a). In the outer peripheral side pit string, three lands M, eachhaving one-clock length, are arrayed with the clamp pits P_(CLMP)in-between, whereas, in the inner peripheral side pit string, threeclamp pits P_(CLMP) are arrayed with the lands M, each having a lengthcorresponding to one clock, in-between.

The clamp operation by the pit array of the clamp area Zc is explainedsubsequently.

The reproducing method for reproducing information signals from theabove-described optical disc is now explained by referring to thereproducing system of the reproducing device shown in FIG. 8.

The reproducing system of the disc reproducing device is made up of anoptical pickup 2 for radiating a laser beam L on an optical disc 1 ofthe first embodiment and detecting the volume of light reflected fromthe recording surface, and a signal processor 3 for reproducing datafrom the playback signals from the optical pickup 2.

The optical pickup 2 is arranged on the same side of the optical disc 1as a spindle motor 4 rotationally driving the optical disc 1 and issubstantially of the same construction as the conventional opticalpickup employed in a compact disc reproducing device, as shown in FIG.9. That is, the optical pickup 2 includes a laser light source 11, acollimator lens 12 for collimating the light outgoing from the laserlight source 11 into a parallel beam, and an objective lens 13condensing the collimated light from the collimator lens 12 forradiating the condensed light on the recording surface 1a of the opticaldisc 1. The optical pickup 2 also comprises a beam splitter 14 arrangedon an optical path between the collimator lens 12 and the objective lens13 for splitting the light reflected from the recording surface 1a ofthe optical disc 1, and a photodetector 15 for detecting the lightvolume of the reflected light Lr split by the beam splitter 14. Theoptical pickup unit finally includes a converging lens 16 arranged on anoptical path between the beam splitter 14 and the photodetector 15 forconverging the reflected by the beam splitter 14.

The objective lens 13 is slightly moved by a two-dimensional actuator 17in a direction towards and away from the optical disc 1 and in theradial direction of the optical disc 1. The two-dimensional actuator 17has a magnetic circuit comprising a focusing coil, a tracking coil and amagnet, all not shown.

The optical pickup 2 is tracking servo controlled so that the trackcenter Tc of the optical disc 1 is scanned by the center of the spot BSof the laser beam L radiated from the laser light source 11.

The laser light source 11 of the optical pickup 2 comprises asemiconductor laser radiating the laser light beam L having the samewavelength as that radiated by the conventional optical pickup. Thecollimator lens 12 collimates the laser beam L from the laser lightsource into a parallel beam which is caused to be incident on the beamsplitter 14.

The beam splitter 14 comprises a half mirror for transmitting theoutgoing light L from the laser light source 11 through the objectivelens 13. The objective lens 13 has a numerical aperture NA equivalent tothat of the conventional optical pickup and converges the outgoing lightL from the laser light source 11 transmitted through the beam splitter14 in order to radiate it on the recording surface 1a of the opticaldisc 1.

On the recording surface 1a of the optical disc 1, as shown in FIGS. 4aand 5a are arrayed a data region Zd and a servo region Zs, made up ofthe servo area Zss and the land Zm, in spatial separation from the dataregion Zd. Above all, in the data region Zd, a pit string correspondingto the recording data is formed on the inner peripheral side of the dataregion Zd, and a corresponding inverted pit string is formed on theouter peripheral side thereof relative to the track center Tc.

Consequently, the reflected light Lr, reflected by the land Zmconstituted solely by the mirror surface, has a light intensitydistribution which becomes transversely symmetrical on the drawingsheet, as shown in FIG. 10a. If there exists in the data region Zd thepit P on the inner peripheral side, while there lacks the pit P on theouter peripheral side, the reflected light Lr has such light intensitydistribution that, due to the diffraction by the pit P on the innerperipheral side, the reflected light deflected towards the right on thedrawing sheet becomes predominant, as shown in FIG. 10b.

Conversely, if there is the pit P on the outer peripheral side of thedata region Zd, while there is no pit P on the inner peripheral side,the reflected light distribution is deflected towards the left morestrongly on the drawing sheet, as shown in FIG. 10c. If there exist pitsP on both the inner and outer peripheral sides of the servo area Zss ofthe servo region Zs, the reflected light Lr has a light intensitydistribution which is transversely symmetrical on the drawing sheet, dueto the diffraction by both of the side pits P, as shown in FIG. 10d).However, the reflected light intensity becomes lower than when there isno pit P.

The reflected light Lr, having the light intensity distribution whichdepends upon the presence or absence of the pits P, is collimated by theobjective lens 13 into a parallel beam which is again caused to fall onthe beam splitter 14 whereby part of the reflected light Lr is split byreflection.

The converging lens 16 is constituted by a cylindrical lens forproducing a focusing error signal by, for example, the astigmaticmethod. By the converging lens 16, the reflected light Lr is convergedon the light receiving surface of the photodetector 15.

The photodetector 15 has its light receiving region divided into foursections 15a, 15b, 15c and 15d, as shown in FIG. 11. The photodetector15 has a far-field pattern on the light receiving surface in which, ifthere is no pit P on the inner peripheral side of the track center Tcand there is the pit P on the outer peripheral side thereof, thesections 15a and 15d become lighter, while the sections 15b and 15cbecome darker due to light diffraction at the pit P, as indicated byhatched lines in FIG. 12a.

Conversely, if there is the pit P on the inner peripheral side and thereis no pit P on the outer peripheral side, as shown in FIG. 12b, thesections 15a, 15d become darker, while the sections 15b, 15c becomelighter. If there exist the pits P on both the inner and outerperipheries, as shown in FIG. 12c, all of the sections 15a to 15d becomedarker. If there exist no pits P on the inner or the outer peripheries,as shown in FIG. 12d, all of the sections 15a to 15d become lighter.

Consequently, there are three signal levels, that is a high level (H), amid level (M) and a low level (L), as the signal levels of the RF signalobtained by summing the detection signals, produced by photo-electricconversion by the four regions 15a to 15d, in association with threecases in which there are no pits P on the inner or outer peripheries(FIGS. 10a and 12d), there exists one pit P on the inner or outerperipheral side (FIGS. 10b, 10c and 12a, 12b) and there exist pits P onboth the inner and outer peripheries (FIGS. 10d and 12c).

The signal processor 3 comprises a head amplifier 21 to which adetection signal Si from the photodetector 15 enters and which performspre-set signal waveshaping based upon the input detection signal Si. Thehead amplifier 21 has plural differential amplifiers built therein andoutputs four kinds of output signals. The first output signal S1 is asum signal of detection signals produced by photo-electric conversion bythe sections 15a, 15d of the photodetector 15 on which fall thereflected lights reflected by the inner peripheral sections of the trackrelative to the track center Tc. The second output signal S1 is a sumsignal of detection signals produced by photo-electric conversion by thesections 15b, 15c of the photodetector 15 on which fall the reflectedlights reflected by the outer peripheral sections of the track relativeto the track center Tc.

The third output signal is a sum signal of detection signals produced byphoto-electric conversion by the four sections 15a to 15d of thephotodetector 15, that is an RF signal SRF. The fourth output signal isa pulse signal Sp outputted by a bottom detection circuit, not shown,which is built into the head amplifier 21. The pulse signal Sp, producedby the bottom detection circuit, is a signal which rises at an outputtiming of the lowermost level signal detected by the bottom detectioncircuit from the RF signal.

The reason the lowermost level signal among the input RF signals SRF isdetected by the bottom detection circuit is that, as describedpreviously, the constructive pattern of the clock mark Mc in the servoarea Zss is such pattern in which there exist pits P on both the innerand outer peripheries with respect to the track center Tc and, if thisportion is scanned by the beam spot BS, the reflected light Lr becomesweakest in light intensity due to light diffraction by the inner andouter peripheral pits P, with the signal waveform of the RF signal SRFproduced at this time being of the lowest signal level. Consequently,generation of the pulse signal Sp which rises at the stage the lowestlevel signal SRF is supplied is equivalent to detection of the clockmark Mc.

The pulse signal Sp is routed to a next stage PLL circuit 22. The PLLcircuit 22 generates a clock signal Sc, which gives the playback timing,based upon the pulse signal Sp from the head amplifier 21 and a clockdetection gate signal Gc from a unique pattern detection circuit 26 aslater explained. The clock signal Sc from the PLL circuit 22 is suppliedto a next stage timing generator (TG) 23 and to an A/D converter 24 aslater explained.

The timing generator 23 generates plural kinds of timing signals, asrequired by various circuits, based upon the clock signals Sc suppliedby the PLL circuit 22. In the present embodiment, these timing signalsare a reference signal Sb supplied to a laser driving circuit 18, asrequired for laser excitation of the laser light source 11 in theoptical pickup 2, a servo clock signal Ss as required for servo controland rotation control of the optical disc 1 and a reference clock Su fordetecting pre-set unique patterns, herein the patterns of the mirrorarea Zm, data region Zd, servo region Zs, clock mark Mc and the addressdata area Za.

The servo clock signal Ss and the reference clock signal Su are of thesame type of signals and represent double signals having a periodone-half the period of the clock signal Sc outputted by the PLL circuit22 and are supplied to the unique pattern detector 27.

The signal processor 3 includes a clamp circuit 25 downstream of thehead amplifier 21. The clamp circuit 25 clamps the levels of the firstoutput signal S₁, second output signal S₂ and the RF signal S_(RF) atthe reference levels and eliminates noise components caused byfluctuations in reflectance of the playback laser beam based upon theinput of the clamp pulse Pc from the unique pattern detection circuit 27as later explained. Since the clamp area Zc in the recording format isallocated immediately after the servo region Zs in each segment, asshown in FIGS. 3 and 6, the clamp operation is performed on the segmentbasis. The clamp area may be detected by forming pits necessarily on theinner peripheral side right after the mirror area Zm.

The signal processor 3 also includes the e.g. 8-bit A/D converter 24 forconverting the first and second output signals S₁, S₂ and the RF signalsS_(RF) entering the A/D converter via the clamp circuit 25, based uponthe output timings of the clock signals Sc from the PLL circuit 22, anda digital equalizer 26 for equalizing the digital signals D₁, D₂ andD_(RF) from the A/D converter 24 by, for example, a 3-tap digitalfilter.

The digital equalizer 26 optimizes the input digital signals D₁, D₂ andD_(RF), depending on the respective densities, as the equalizationcoefficients, for providing the optimum error rates, in order to produceoptimized digital signals D₁ ', D₂ ' and D_(RF) ', and calculates thedifference between the digital signals D₁ ' and D₂ ' associated with thefirst and second output signals S₁ and S₂ in order to produce apush-pull signal D_(pp).

The signal waveforms of the push-pull signal Dpp prepared by the digitalequalizer 26 and the RF signal S_(RF) from the head amplifier 21 areshown in FIGS. 4b, 4c and in FIGS. 5b, 5c, respectively. Although thepush-pull signal Dpp is formulated in the present embodiment as digitalsignals, it is described herein as an analog signal for facilitatingwaveform comparison with the analog RF signal S_(RF).

The push-pull signal waveform and the RF signal waveform, shown in FIGS.5b add 5c, are waveforms obtained on reproducing the pit string and theinverted pit string of the recording data area Zw in the data region Zdshown in FIG. 5a. Referring to FIG. 5, since there necessarily existsthe mirror surface M on the outer peripheral side in the recording dataarea Zw with respect to the track center Tc, if there exists the pit Pon the inner peripheral side, whereas, if the inner peripheral side isthe mirror surface M, there exists the pit P on the outer peripheralside, so that the signal level of the RF signal S_(RF) is the mid level(M).

On the other hand, the push-pull signal Dpp is of a signal waveformwhich becomes zero at a boundary between the pit P and the mirrorsurface M in the pit string or in the inverted pit string and which isdeviated in the (-) direction and in the (+) direction when there is thepit P in the inner peripheral side and in the outer peripheral side,respectively.

The push-pull signal waveform and the RF signal waveform, shown in FIGS.4b and 4c, are the waveforms obtained on reproducing the pit string andthe inverted pit string in the servo region Zs and the near-by regionand, above all, the playback waveforms obtained on scanning the thirdtrack T₃ by the beam spot BS. It is seen from these figures that, sincethe mirror area Zm is constituted solely by the mirror surface M, andthere exists no pit P on the inner or outer peripheral side of the trackcenter Tc, the RF signal S_(RF) for the mirror area Zm has a high signallevel (H).

In the portion of the servo area Zss where there is the servo mark Ms,since there is the pit P on the inner or outer peripheral side, thesignal level of the RF signal S_(RF) for the servo mark Ms is the midlevel (M). In the portion of the servo area Zss where there is the clockmark Mc, since there exist the pits P on the inner and the outerperipheral sides, the signal level of the RF signal S_(RF) for the clockmark Mc is the low level (L).

On the other hand, the push-pull signal Dpp is of a signal waveformwhich becomes zero at the portions of the track registering with themirror area Zm and the clock mark Mc and which is deviated in the (-)direction and in the (+) direction at the portions of the servo area Zssregistering with the servo mark Ms if there is the pit P in the innerperipheral side and in the outer peripheral side, respectively.

Downstream of the digital equalizer 26 are connected the above-mentionedunique pattern detection circuit 27, a servo error signal generator 28,a threshold value calculating circuit 29 and a partial response PR(1, 1) detection circuit 30.

The push-pull signal Dpp and the digital RF signal D_(RF) from thedigital equalizer 26 enter the unique pattern detection circuit 27which, based upon the push-pull signal Dpp, digital RF signal D_(RF) andthe reference clock signal Su from the timing generator 23, formulateand output five different kinds of unique pattern detection signals Gm,Gd, Gs, Gc and Ga and the clamp pulse Pc.

If assumed that the third track T₃ is scanned by the beam spot BS of theplayback laser beam, the unique pattern detection gate signal, which isgenerated at the unique pattern detection circuit 27 is made up of amirror area detecting gating signal Gm, which goes high only for theportion of the track registering with the mirror area Zm, a data regiondetecting gating signal Gd which goes high only for the portion of thetrack registering with the data region Zd, a servo region detectinggating signal Gs which goes high only for the portion of the trackregistering with the servo region Zs, a clock detecting gating signal Gcwhich goes high only for the portion of the servo region Zs of the trackregistering with the clock mark Mc and an address data detecting gatingsignal Ga which goes high only for the portion of the data region Zd ofthe track registering with the address data area Za, as shown in FIG.4d.

These detection gate signals Gm, Gd, Gs, Gc and Ga are prepared by theunique pattern detection circuit 27 in the following manner. That is,the unique pattern detection circuit 27 first detects the clock mark hlcin the servo area Zss of the servo region Zs and, based upon thedetection of the clock mark Mc, formulates the clock detecting gatingsignals Gc, which then is outputted to the PLL circuit 22. The PLLcircuit 22 formulates and outputs reference clock signals for the systemoperation based upon the input clock detecting gating signal Gc and thepulse signal Sp indicating the clock mark Mc from the head amplifier 21.The clock signal Sc is converted in the timing generator 23 to thereference clock signal Su (and Ss) which is routed to the unique patterndetection circuit 27.

Based upon the above-mentioned servo region detecting gating signal Gsand the reference clock signal Su from the timing generator 23, theunique pattern detection circuit 27 formulates the remaining componentsof the unique pattern detecting gating signal, namely the mirror areadetecting gating signal Gm, data region detecting gating signal Gd andthe address data area detecting gating signal Ga. The data regiondetecting gating signal Gd is produced for all of the segments, whereasthe address data area detecting gating signal Ga is produced only forthe address and address/data segments.

Of these detection gate signals Gm, Gd, Gs, Gc and Ga, the mirror areadetecting gating signal Gm, servo region detecting gating signal Gs andthe clock detecting gating signal Gc are supplied to the servo errorsignal generator 28, whereas the data region detecting gating signal Gdand the address data area detecting gating signal Ga are supplied to anarithmetic-logical unit 31 as later explained.

The unique pattern detection circuit 27 also generates, from thepush-pull signal produced on reproducing the clamp pits P_(CLMP) makingup the clamp area Zc, a clock pulse Pc which rises during zero detectionas the push-pull signal is changed from the (-) polarity to the (+)polarity, and outputs the generated clock pulse Pc.

Referring to the recording format shown in FIGS. 3 and 6, three clamppits P_(CLMP) are formed in the inner peripheral pit string with themirror surfaces or lands M in-between, and three mirror surfaces orlands M are formed with clamp pits P_(CLMP) in-between, so that thereexist two timings at which the push-pull signal is changed in polarityfrom the (-) polarity to the (+) polarity and hence two clamp pulses Pcare outputted per segment.

The clamp pulses Pc, outputted from the unique pattern detection circuit27, are routed to the clamp circuit 25, as mentioned previously.

A defect indicating flag is logically allocated in a memory built in theinside of the unique pattern detection circuit 27. The defect indicatingflag is constructed of at least one bit and is set to "1" via an innercontrol unit when any one of the unique patterns results in failure. Ifdetection of all of the unique patterns is achieved successfully, theflag is set to "0". The bit information Df of the defect indicating flagis routed to the detection circuit 30.

The threshold value calculating circuit 29 fetches the reference pitinformation recorded in the header of each sector from the push-pullsignal Dpp and the digital RF signal from the digital equalizer 26, andcalculates threshold value data D_(th) from the thus fetched referencepit information. The threshold value data D_(th) is routed to thedetection circuit 30.

Based upon the threshold value data D_(th) from the threshold valuecalculating circuit 29, the detection circuit 30 carries out detectionof the PR (1, 1) of the digital RF signal D_(RF) and the PR (1, 1) ofthe push-pull signal D_(PP). It should be noted in particular that twokinds of detection techniques are stored as algorithm in a programmemory of the detection circuit 30. One of the techniques is theso-called bit-by-bit system algorithm of carrying out detection based onthe threshold value data D_(th) from the threshold value calculatingcircuit 29 and the other technique is the Viterbi decoding basedalgorithm.

Which of these techniques should be employed depends upon design data,such as the recording density, of the optical disc 1 being reproduced.That is, detection of the push-pull signal D_(PP) and the RF signalD_(RF) by the bit-by-bit system is inferior to that by Viterbi decodinginsofar as suppression of the inter-symbol interference is concerned.However, it, is a technique which may be advantageously employed inreproduction of the optical disc 1 of a lower recording density.Detection of the push-pull signal D_(PP) and the RF signal D_(RF) by theViterbei decoding system is higher in its detection capability than thatby the bit-by-bit system, and is capable of improving the S/N ratio by3.6 dB, so that it is a detection technique which may be advantageouslyemployed for reproducing the high recording density optical disc 1.

The push-pull signal D_(pp) and the RF signal D_(RF), detected by thedetection circuit 30 by PR (1, 1), are routed to a downstream sidearithmetic-logical unit 31.

Based upon the push-pull signal D_(PP) and the RF signal D_(RF) from thedetection circuit 30 by PR (1, 1), the arithmetic-logical unit 31reproduces the data recorded on the optical disc 1. Thearithmetic-logical unit 31 has two types of the arithmetic-logicalcircuits, namely an address data processor 32 and a recording dataprocessor 33, and a control circuit 34 controlling these processors 32,33 and combining playback data from the processors 32, 33 into a soleoutput data string. Gating circuits, not shown, are provided at aninitial stage of each of the processors 32 and 33. The unit 31 alsoincludes a memory 35 for transiently storing data responsible for errorgeneration during reproduction via the control circuit 34.

The address data processor 32 and the recording data processor 33 areexplained in detail. First, the address data processor 32 has a gatingcircuit, not shown, connected to an initial stage thereof which isopened when both the data region detecting gating signal Gd and theaddress data region detecting gating signal Ga from the unique patterndetection circuit 27 are both at a high level, in order to permit thepush-pull signal D_(PP) and the RF signal D_(RF) from the detectioncircuit 30 to be routed to the address data processor 32.

That is, in the present embodiment, the push-pull signal D_(PP) and theRF signal D_(RF) are caused to enter the address data processor 32selectively, such that the push-pull signal D_(PP) and the RF signalD_(RF) are allowed to enter the address data processor 32 only when boththe data region detecting gating signal Gd and the address data regiondetecting gating signal Ga from the unique pattern detection circuit 27are at a high level.

That is, the push-pull signal D_(pp) and the RF signal D_(RF), producedby reproducing the access code and the data code recorded in eachaddress data area Za of the address segment and the address/datasegment, are caused to enter the address data processor 32. The addressdata processor 32 reproduces the access code and the sector coderecorded on the optical disc 1, based upon the output timing of theclock signals Sc from the PLL circuit 22, by forming a matrix of theinput push-pull signal D_(PP) and the RF signal D_(RF).

The following Table 1 shows the matrix for reproducing data recorded onthe optical disc 1.

                  TABLE 1                                                         ______________________________________                                                     push-pull signal                                                              +         o     -                                                ______________________________________                                        RF signal    H     x           O   x                                                       M     O           O   O                                                       L     x           O   x                                          ______________________________________                                    

In the above Table 1, the polarities of the push-pull signals D_(PP) andthe signal levels of the RF signals D_(RF) are arrayed in the rows andcolumns, respectively. Referring to Table 1, if the RF signal D_(RF) isa high (H) level signal, there is no pit P in the beam spot BS and hencethe push-pull signal D_(PP) cannot be produced, so that the state (+) or(-) of the push-pull signal D_(PP) cannot be assumed. Consequently,there cannot exist a state of the RF signal D_(RF) being at a high (H)level and the push-pull signal D_(PP) being of the (+) or (-) polarity.

If the RF signal D_(RF) is of a mid-level value (M), there exists onlyone pit P within the beam spot BS, or there exists a boundary betweenthe pit P and the land M in the inner peripheral pit string or in theouter peripheral inverted pit string. In the former and latter cases,the push-pull signal D_(PP) assumes the state of "+" or "-" or the stateof 0, respectively.

On the other hand, if the RF signal D_(RF) is of the low (L) level,there exist two pits P in the beam spot BS, and hence the push-pullsignal D_(PP) cannot be produced, so that the state of the push-pullsignal D_(PP) being "+" or "-" cannot be assumed. Consequently, therecannot exist the state of the RF signal D_(RF) being of the low (L)level and the push-pull signal D_(PP) being of the "+" or "-" polarity.

By the data region detecting gating signal Gd and the address datadetecting gating signal Ga, the set of the push-pull signal Dpp and theRF signal D_(RF) in the data region Zd enter the address data processor32. The processor 32 formulates data consisting in the input RF signaland the push-pull signal in which the push-pull signal is of zeropolarity and is "1" or "0" when the RF signal is of a low level (L) orotherwise, respectively, in order to reproduce the access code and thesector code in each address data area Za in the address segment and inthe sector code.

On the other hand, the gating circuit, not shown, connected to theinitial stage of the recording data processor 33, is opened when onlythe data region detection gate signal Gd from the unique patterndetection circuit 27 is of a high level, in order to permit thepush-pull signal D_(PP) and the RF signal D_(RF) to be routed from thedetection circuit 30 into the recording data processor 33.

That is, in the present embodiment, the push-pull signal D_(PP) and theRF signal D_(RF) are caused to enter the recording data processor 33selectively, such that the push-pull signal D_(PP) and the RF signalD_(RF) are inputted to the recording data processor 33 when only thedata region detecting gating signal Gd from the unique pattern detectioncircuit 27 is of a high level.

That is, the push-pull signal D_(PP) and the RF signal D_(RF), producedby reproducing recording data recorded in each recording data area Zw ofthe data segment and the address/data segment, are inputted to therecording data processor 33. The recording data processor 32 reproducesthe recorded data on the optical disc 1, based upon the output timing ofthe clock signals Sc from the PLL circuit 22, by forming a matrix of theinput push-pull signal D_(PP) and the RF signal D_(RF).

Specifically, since the signal level of the RF signal D_(RF) enteringthe recording data processor 33 is usually of the mid level (M), datareproduction is actually performed based upon the combination for therow of the mid-level value (M) of the RF signal D_(RF) in Table 1. Thatis, data which becomes logical "1" and "0" when the push-pull signalD_(pp) becomes zero or is of the "+" or "-" level are produced, by wayof reproducing recording data recorded on the optical disc 1.

The address data reproduced by the address data processor 32 and therecording data processor 33 are routed over an interface bus, such as anSCSI bus 36, so as to be stored in an array variable storage region of amain memory or an auxiliary memory within the computer.

In addition to the above-mentioned components, a data defect circuit,not shown, for detecting the recording data, is built in the recordingdata processor 33. The recording data defect circuit generates an errorsignal set when the signal level of the RF signal D_(REF) entering therecording data processor 33 is not the mid level (M), and routes thegenerated error signal to the control circuit 34. Based upon theinputting of the error signal Set from the recording error processor 33,the controller 34 stores the defect recording data from the recordingerror processor 33 in the pre-set array variable region of the memory 35along with the corresponding address data. The address data ismaintained in the memory 35 until re-delivery of the data concerning thestored address. If the address data which is the same as the storedaddress is re-delivered, the re-delivered data is transmitted to a hostcomputer over an interfacing bus 36 along with the address data.

If the information Df of the defect indicating flag supplied from theunique pattern detection circuit 27 to the detection circuit 30indicates a defect, as when it is a logical "1" data, detection by PR(1, 1) of the RF signal D_(RF) is again performed, or a data signalindicating "warning" is outputted to the control circuit 34 of thearithmetic-logical unit 31. If supplied with the data signal "warning"from the detection circuit 30, the control circuit 34 causes thereproduced data and the addresses of the data to be stored in a pre-setarray variable region of the memory 35 built in the arithmetic-logicalunit 31. The data concerning the address is maintained in the memory 35until re-delivery of data concerning the same address as the storedaddress. In case of re-delivery of the data concerning the same addressas the stored address, the re-delivery data is transmitted to the hostcomputer over the interfacing bus 36 along with the address.

Next, the servo control related constitution is explained. Three gatecircuits, not shown, are built into an initial stage of the servo errorsignal generator 28, and are designed to be opened by high-level inputsof the mirror area detecting gating signal Gm, servo region detectinggating signal Gs and the clock detecting gating signal Gc, respectively.

The servo error signal generator 28 has built therein a spindle servoerror signal generating circuit, not shown, for generating a spindleservo error signal Es for controlling the spindle motor 4, a focusingerror signal generating circuit, not shown, for generating a focusingerror signal Ef for servo controlling the focal length of the laser beamL by the objective lens 13, and a tracking error signal generatingcircuit, not shown, for generating a tracking error signal Et for servocontrolling the tracking of the beam spot BS,

Downstream of the servo error signal generating circuit 28 is connecteda servo circuitry 38 via a phase compensator 37. The servo circuitry 38has built therein a spindle servo control circuit, not shown, forservo-controlling the spindle motor 4, a focusing servo controllingcircuit, not shown, for servo controlling the focal length of the laserbeam L by the objective lens 13 and a tracking servo controllingcircuit, also not shown, for servo controlling the tracking of the beamspot BS.

To the servo error signal generating circuit 28, digital RF signalsD_(RF) ', processed with digital equalization by the digital equalizer26, are caused to enter selectively via the three gating circuits. Ofthese three gating circuits, the first gating circuit is responsive tothe high-level clock detecting gating signal Gc to transmit the digitalRF signals D_(RF) ' to the spindle servo error signal generatingcircuit, while the second gating circuit is responsive to the high levelmirror area detection signal Gm to transmit the digital RF signalsD_(RF) ', as calculated based upon astigmatic aberration, to thefocusing error signal generating circuit. The third gating signalresponsive to the high level servo region detecting gating signal Gs totransmit the digital RF signals D_(RF) ' to the tracking errorgenerating circuit.

The spindle servo error signal generating circuit generates the clockpulse signal Es which rises based upon the output timing of the inputdigital RF signals D_(RF) '. The clock pulse signal Es is routed via thephase compensation circuit 37 to the spindle servo control circuitwithin the servo circuitry 38 via the phase compensation circuit 38. Thespindle servo control circuit servo controls the spindle motor 4 basedupon the clock pulse signal Es supplied thereto for running the opticaldisc 1 in stable rotation in accordance with, for example, the CAVsystem.

The focusing error signal generating circuit calculates the differencebetween the signal level of the input digital RF signals D_(RF) ' and apre-set mirror level and routes the difference signal Ef to the focusingservo controlling circuit of the servo circuitry 38 via the phasecompensation circuit 37. The focusing servo controlling circuit causesthe control current to flow in a focusing coil of a two-dimensionalactuator 17 so as to reduce the signal level of the supplied differencesignal Ef in order to shift the objective lens 13 in a direction towardsand away from the optical disc 1 by way of performing focal pointadjustment of the objective lens 13.

Turning to the tracking servo control, the arraying pattern of the servomarks Ms making up the servo area Zss is changed at an interval of threetracks. If an imaginary track T, is assumed to be present between twoneighboring tracks T₁ and T₂ the arraying pattern of the servo marks Msbecomes different for every three tracks inclusive of the imaginarytrack T. Consequently, the digital RF signals D_(RF) ', entering thetracing error signal generating circuit from the digital equalizer 26,is the three-phase signal having different phases with respect to thefirst track T₁, imaginary track T and the second track T₂.

If a track jump is made, such that the beam spot BS of the laser beam Lis swept obliquely relative to the track, by way of seeking, the digitalRF signals D_(RF) ', inputted from the digital equalizer 26, iscomprised of three sine waves dephased 120° relative to one another,that is a first RF signal RFa, entering at a timing corresponding to theregion A, a second RF signal RFb, entering at a timing corresponding tothe region B and a third RF signal RFb entering at a timingcorresponding to the region C, as shown in FIG. 13a.

As shown in FIG. 14, the tracking error signal generating circuitincludes sample-and-hold circuits 47a, 47b and 47c for sample-holdingthe digital RF signals D_(RF) ' from the digital equalizer 26 withsampling clocks dephased 120° relative to one another, differentialamplifiers 41a, 41b and 41c for finding the differences of the first,second and third RF signals RFa, RFb and RFc from the sample-and-holdcircuits 47a to 47c, a multiplexor 42 for selecting one of outputs ofthe differential amplifiers 41a, 41b and 41c, comparators 43a, 43b and43c for detecting the polarities of the outputs of the differentialamplifiers 41a, 41b and 41c and an arithmetic-logical circuit 44 forcontrolling the multiplexor 42 based upon a pre-set arithmetic-logicaloperation on the outputs of the comparators 43a, 43b and 43c.

Referring to FIG. 13b, the differential amplifier 41a subtracts thesecond RF signal RFb from the third RF signal RFc to produce thetracking error signal TRa, indicated by a broken line. The differentialamplifier 41b subtracts the third RF signal RFb from the first RF signalRFa to produce the tracking error signal TRb, indicated by a brokenline. The differential amplifier 41c subtracts the first RF signal RFafrom the second RF signal RFb to produce the tracking error signal TRc,indicated by a broken line.

Thus the tracking signals TRa, TRb and TRc are sine waves dephased by120° relative to one another and facing phase-lead of 90° with respectto the RF signals TRa, TRb and TRc. The tracking error signals TRa, TRband TRc thus produced are routed to the multiplexor 42 and tocomparators 43a, 43b and 43c.

Meanwhile, the tracking error signals TRa, TRb and TRc are of a dynamicrange which is due to the diffraction at the servo pit Ms, as shown inFIG. 4a, and which may be increased in magnitude as compared to that ofthe conventional optical disc. In other words, the tracking errorsignals TRa, TRb and TRc having a superior S/N ratio may be obtained.

Referring to FIG. 13c, the comparators 43a, 43b and 43c detect thepolarities of the tracking error signals TRa, TRb and TRc and, if thesignal level is positive, the comparators generate polarity signals Pa,Pb and Pc, which are routed to the arithmetic-logical circuit 44. Thearithmetic-logical circuit 44 calculates control signal Ca, Cb and Cc,dephased 120° relative to one another, in accordance with the followingequations (1) to (3):

    Ca=Pc∩INV(Pb)                                      (1)

    Cb=Pa∩INV(Pc)                                      (2)

    Cc=Pb∩INV(Pa)                                      (3)

and controls the multiplexor 42 for selecting the tracking error signalsTRa, TRb and TRc when the control signals Ca, Cb and Cc are logical "1",respectively.

In the equations (1) to (3), the symbols ∩ and INV () stand for logicalproduct and negative logic, respectively.

The multiplexor 42 outputs a three-phase tracking error signal, which isproduced by periodically changing over the three-phase tracking errorsignals TRa, TRb and TRc, as shown by a solid line in FIG. 13(b). Thetracking error signal is routed to the phase compensation circuit 37.

The tracking error signal, phase-compensated by the phase-compensationcircuit 37, is routed to a tracking servo circuit of the servo circuitry38. The tracking servo control circuit causes the control current toflow through a tracking coil of the two-dimensional actuator 17, untilthe signal level of the supplied tracking error signal becomes equal tozero, thereby causing the objective lens 13 to be moved along the radiusof the optical disc land causing the center of the beam spot BS to scanthe track center Tc, by way of performing tracking adjustment.

As shown in FIG. 13(b), the tracking error signal outputted from thetracking error signal generating circuit is devoid of an area 202 otherthan an area which permits of stable tracking servo, as alreadyexplained with reference to FIG. 24, so that perpetually stable trackingservo may be achieved.

In addition, if assumed that the tracking error signal shown in FIG. 13bis the signal produced when the laser beam L is seeking from the outerperiphery towards the inner periphery of the disc, the waveform is onewhich retrogresses along the time scale. Thus, for seeking from theinner periphery towards the outer periphery, the signal level isperpetually increased in a range experiencing the continuously changingsignal level, whereas, for seeking from the outer periphery towards theinner periphery, the signal level is perpetually decreasing in the rangeexperiencing the continuously changing signal level.

Consequently, it becomes possible to detect the direction of movement ofthe spot BS of the laser beam L depending upon the direction of levelchanges in the range having the continuously changing signal level. Inother words, it becomes possible with the present reproducing system toproduce the tracking error signals containing the information concerningthe seek direction.

The tracking error signal generating circuit may be simplified incircuit construction because there is no necessity of providing dividersor memories as required in the conventional playback system.

Referring to FIG. 15, a modified circuit construction of theabove-mentioned tracking error signal generating circuit is explained.Meanwhile, the circuit having the same function as that of the trackingerror signal generating circuit shown in FIG. 14 is indicated by thesame reference numerals and the corresponding description is omitted foravoiding redundancy.

Referring to FIG. 15, the tracking error signal generating circuithaving the modified circuit construction includes sample-and-holdcircuits 47a, 47b and 47c for sample-holding the RF signals D_(RF) 'from the digital equalizer 26, and differential amplifiers 41a, 41b and41c for finding the differences among the RF signals RFa, RFb and RFcfrom the digital equalizer 26, outputted from the sample-and-holdcircuits 47a, 47b and 47c. The tracking signal generating circuit alsoincludes an additive node 51 for summing the RF signals RFa, RFb and RFcfrom the digital equalizer 26 and differential amplifiers 52a, 52b and52c for subtracting an output of the additive node 51 from the RFsignals RFa, RFb and RFc from the digital equalizer 26. The trackingerror signal generating circuit also includes first A/D converters 53a,53b and 53c for converting the outputs of the differential amplifiers41a, 41b and 41c into digital signals, and second A/D converters 54a,54b and 54c for converting the outputs of the differential amplifiers52a, 52b and 52c into digital signals. The tracking signal generatingcircuit also includes a ROM 55 for outputting the tracking errorsignals, using the tracking error signals, supplied from the first andsecond A/D converters 53a, 53b and 53c and 54a, 54b and 54c, asaddresses, and a D/A converter 56 for converting the tracking errorsignals, supplied as digital signals from the ROM 55, as digitalsignals. The tracking signal generating circuit finally includescomparators 43a, 43b and 43c for detecting the polarities of the outputsof the differential amplifiers 41a, 41b and 41c and anarithmetic-logical circuit 44 for controlling the ROM 55 based upon apre-set arithmetic-logical operation on the outputs of the comparators43a to 43c.

Between the ROM 55 and the D/A converter 56, there are connected a firstlatch circuit 58a for fetching the tracking error signal from the ROM 55based upon the input of the servo clock signal Ss from the timinggenerator 23 for holding the tracking error signal as the currenttracking error signal, a second latch circuit 58b for fetching thetracking error signal outputted from the first latch circuit 58a basedupon the input of the servo clock signal Ss from the timing generator 23for holding the tracking error signal as the temporally previoustracking error signal and a switching circuit 59 for selectivelycoupling outputs of the first latch circuit 58a and the second latchcircuit 58b to the downstream side D/A converter 56. The switchingcircuit 59 is normally operated for setting a movable contact of aswitch 60 towards the first latch circuit 58a for routing the currenttracking error signal to the D/A converter 56.

The differential amplifiers 41a, 41b and 41c generate tracking errorsignals TRa, TRb and TRc, which are sine waves dephased 120° relative toone another and which have a phase lead of 90° with respect to theassociated RF signals RFa, RFb and RFc, respectively, as shown in FIG.13b, and transmit these tracking signals TRa, TRb and TRc to the firstA/D converters 53a, 53b and 53c and to the comparators 43a, 43b and 43c,respectively.

The first A/D converters 53a, 53b and 53c translate the tracking errorsignals TRa, TRb and TRc to digital signals and route the digitizedtracking error signals TRa, TRb and TRc to the ROM 55 as addresses.

The additive node 51 sums the RF signals RFa, RFb and RFc from thedigital equalizer 26 and transmits the sum, that is the mean value ofthe three-phase alternating current RF signals RFa, RFb and RFc, to thedifferential amplifiers 52a, 52b and 52c, as shown in FIG. 13a. It isnoted that the mean value is a median value which is a constant C.

The differential amplifiers 52a, 52b and 52c subtract the mean value Cfrom the RF signals RFa, RFb and RFc, that is, shift the RF signals RFa,RFb and RFc, by the mean value C, in order to provide the RF signalsRFa, RFb and RFc freed of the dc component, and transmit the RF signalsRFa, RFb and RFc, freed of the dc component to the second A/D converters54a, 54b and 54c, respectively.

The second A/D converters 54a, 54b and 54c convert the RF signals freedof the dc component to digital signals and transmit the digitized RFsignals to the ROM 55 as addresses.

In this manner, the digitized tracking error signals TRa, TRb and TRcand the RF signals RFa, RFb and RFc are supplied to the ROM 55, in whichthere is pre-stored a data table satisfying a certain relation asconcerns with the tracking error signals TRa, TRb and TRc and the RFsignals RFa, RFb and RFc. Thus the ROM 55 outputs the tracking errorsignal shown by a solid line in FIG. 13e, using the tracking errorsignals TRa, TRb and TRc and the RF signals RFa, RFb and RFc as theaddresses.

Specifically, if the displacement of the beam spot BS of the laser beamL from the track center Tc is x, the track pitch is p, and the RF signalRFa from the digital equalizer 25 is V_(QA), which V_(QA) is representedby the following equation (4):

    V.sub.QA =K.sub.1 cos (2πx/p)+C                         (4)

an output v_(QA) of the differential amplifier 52a is represented by thefollowing equation (5):

    v.sub.QA =V.sub.QA -C=K.sub.1 cos (2πx/p)               (5)

On the other hand, since the tracking error signal TRa from thedifferential amplifier 41a is dephased by 90° from the RF signal RFa,the tracking error signal TRa, indicated as v_(PA), is represented bythe following equation (6):

    v.sub.PA =K.sub.2 sin (2πx/p)                           (6)

where K₂ /K₁ =1.

From these equations (5) and (6), a signal v_(x) indicating thedisplacement x is obtained from the following equation (7):

    v.sub.x =(p/2π) tan.sup.-1 (v.sub.PA /v.sub.QA)         (7)

It is noted that, since the signal v_(x) is proportional as a principleto the displacement x in a range of |x|<(p/4), values on a straight lineextended from a segment of a line in the range of |x|<(p/4) are storedin a range of from |x|<(3p/2) in the ROM 55, and the data table islooked up using the digitized tracking error signal TRa (v_(pa)) and theRF signal RFa (v_(QA)) in order to obtain the tracking error signal TRa₁shown by a broken line in FIG. 13e.

Similarly, data tables for the remaining tracking error signals TRb₁,TRc₁ are stored and read out using the digitized tracking error signalTRb and the RF signal RFb or using the tracking error signal TRc and theRF signal RFc.

The above-mentioned control signals Ca, Cb and Cc from thearithmetic-logical circuit 44, shown in FIG. 13d, are supplied asreadout control signals to the ROM 55, to which a changeover controlsignal between the normal mode and the lock mode is also supplied via aterminal 57. The tracking error signals TRa₁, TRb₁ and TRc₁ are selectedfor the control signals Ca, Cb and Cc equal to logical "1",respectively, for outputting a tracking error signal which is obtainedby periodically changing over the three-phase tracking error signalsTRa₁, TRb₁ and TRc₁, dephased relative to one another, as shown in FIG.13e.

One of the tracking error signals TRa₁, TRb₁ and TRc₁ is selected andoutputted during the lock mode without regard to the control signals Ca,Cb and Cc.

In the tracking error signal generating circuit of the modified circuitconstruction, a defect detection circuit 61 is connected downstream ofthe additive node 51. If the seek operation is going on as normally, thesum of the first, second and third RF signals RFa, RFb and RFc (meanvalue C) shown in FIG. 13(a) is set at a pre-set reference value.Consequently, if the above-mentioned sum (mean value) of the first,second and third RF signals RFa, RFb and RFc is not fixed at the pre-setreference value, some defect is occurring in the seek operation.

The defect detection circuit 61, adapted for detecting such defects inthe seek operation, compares the sum value (mean value) from theadditive node 51 to a reference value, and outputs a changeover signalStr having a signal waveform corresponding to the result of comparisonto the switching circuit 59. That is, if the result of comparisonexceeds a pre-set allowable range, the detection circuit 61 deems thatthere is some defect, and outputs a high-level signal as a changeoversignal Str. Conversely, if the result of comparison is not in excess ofthe allowable range, the detection circuit deems that there is no defectand outputs a low-level signal as the changeover signal Str. Based uponthe input of the changeover signal Str from the defect detection circuit61, the switching circuit 59 changes over the switch 60 towards the sideof the second latch circuit 58b, during the time the changeover signalStr is at the high level, in order to transmit the directly previoustracking error signal to the D/A converter 56. The switching circuitselects the switch 60 towards the side of the first latch 58a during thetime the changeover signal Str is in the low level, which is the normalcondition in order to transmit the current tracking error signal to theD/A converter 56.

The track error signal read out from the ROM 55 as described above isconverted by the D/A converter 56 to analog signals which are routed viathe phase compensation circuit 37 to the tracking servo control circuitof the servo circuitry 38 in the same manner as in the previousembodiment. As a result, the tracking error signal is devoid of therange 202 other than the range 201 corresponding to the stable trackingservo control (see FIG. 24) so that stable tracking servo control may beperpetually achieved. Besides, the tracking error signal containing theinformation as to the seek direction may also be obtained.

Referring to FIG. 4(a), the operation of track jump is explained. Iftrack jump is made from the first track T₁ to the second track T₂ as thefirst track T₁ is under tracking control, the control signal is suppliedvia terminal 57 in order to change over the readout from the ROM 55compulsorily from the tracking error signal TRa₁ to the tracking errorsignal TRb₁. The lock mode is then set for which tracking error signalscannot be changed over irrespective of the control signals Ca, Cb or Cc.

Specifically, when the tracking is on the first track T₁, the beam spotBS of the laser beam is at the track center Tc of the first track T₁corresponding to the zero-crossing point X_(A) of the tracking errorsignal TRa₁, for the control signal Ca being in the logical "1", asshown in FIG. 16.

If, in this state, the signal readout from the ROM 55 is switched to thetracking error signal TRb₁, without regard to the control signals Ca, Cband Cc, a level L signal is outputted from the ROM 55. Thetwo-dimensional actuator 17 shifts the beam spot BS of the laser beam Ltowards the track center of the imaginary track T, corresponding to thezero-crossing point X₀ of the tracking error signal TRb₁, so that thelevel L will become smaller.

Subsequently, a control signal is applied via terminal 57, in the samemanner as described above, to force a switching of the readout from theROM 55 from the tracking error signal TRb₁ to the tracking error signalTRc₁. Since the spot BS of the laser beam L is at the track center ofthe imaginary track T corresponding to the zero-crossing point X_(B) ofthe imaginary track T, if the readout from the ROM 55 is switched sothat the tracking error signal TRc₁ is read out, irrespective of thecontrol signals Ca, Cb or Cc, the level L signal is outputted from theROM 55. The two-dimensional actuator 17 shifts the spot BS of the laserbeam L towards the track center Tc of the second track T₂ correspondingto the zero-crossing point X_(c) of the tracking error signal TRc₁, sothat the level L becomes smaller. This completes the track jump.

With the above-described tracking servo control, track jump may beperformed in the closed loop condition, while it is unnecessary to openthe tracking servo loop in contradistinction from the conventionalpractice. In other words, there is no necessity of providing circuitcomponents (electronic devices) for opening the tracking servo controlloop, thereby enabling cost reduction.

In addition, by providing the lock mode as described above, the capturerange in the tracking servo control may be increased. For example, ifswitching is made from the tracking error signal TRa₁ to the trackingerror signal TRb₁ for establishing the lock mode and subsequently somedisturbance has occurred, a stable track jump may nevertheless beperformed due to the presence of an operating margin as shown in FIG.16.

The production process for the optical disc 1 of the above-describedembodiment is explained. The production process is roughly divided intoa mastering process and a replication process.

The mastering process is a process up to completion of a metal maserplate (stamper) employed in the duplication process. The duplicationprocess is a process of producing a large quantity of optical discs 1 asreplicas of the stamper.

In the mastering process, a photoresist is applied to a ground glasssubstrate, on which data are recorded as the bit string information bylight exposure, using a laser beam, by way of performing laser cutting.The recording data need to be prepared in advance by a process known aspre-mastering.

After the end of the cutting, pre-set operations, such as development,are performed, after which the information is transcribed on the metalsurface by, for example, electro-casting, in order to complete thestamper required in duplicating the optical discs.

Then, using the stamper, produced as described above, the information istranscribed on a resin substrate by, for example, the injection method.After forming the light reflective film thereon and performing finishingoperations, an ultimate product is completed in a known manner.

Consequently, the information recording device for recording theinformation on the optical disc 1 is a so-called laser cutting devicefor forming the pit string information on the photoresist on the glasssubstrate using the laser beam.

Referring to FIG. 17, the laser cutting device of the present embodimentis explained.

As shown therein, the laser cutting device includes an optical unit, 72for radiating the laser beam L on the glass substrate 71 coated with thephotoresist for forming the pit, string information on the photoresistsurface, a rotating driving unit 73 for rotationally driving the glasssubstrate 71 and a signal processor 74 for converting the recording datainto the pit string data information and controlling the optical unit 72and the rotating driving unit 73.

The optical unit 72 includes a laser light source 75, a light modulator76 for modulating light intensity of the outgoing light L from the lightsource 75 based upon the pit string data information, a prism 77 forbending the optical axis of the modulated light beam from the lightmodulator 76 and an objective lens 78 for converging the modulated andreflected light, beam from the prism 77 on the photoresist surface ofthe glass substrate 71.

The rotating driving unit 73 includes a motor 79 for rotationallydriving the glass substrate 71 in accordance with the CAV system, an FGgenerator 80 for generating pulses (FG pulses) for detecting therotational speed of the motor 79, a slide motor 81 for sliding the glasssubstrate 71 along its radius, and a servo controller 82 for performingvarious servo control operations, such as tracking control of theobjective lens 78 or rotational speed control of the slide motor 81.

The signal processor 74 includes a data formatting circuit 91 forappending code data such as error correction code data to the sourcedata Ds from a computer to form recording data Dw, an arithmetic-logicalunit 92 for performing pre-set arithmetic-logical operations on therecording data Dw from the data formatting circuit 91 for converting therecording data to pit string information data Dp and a data invertingcircuit 93 for inverting the logical data of constituent bits of the pitstring information data Dp outputted from the arithmetic-logical circuit92 for outputting inverted pit string data iDp. The signal processor 74also includes a selector 94 for selecting one of the pit stringinformation data Dp from the arithmetic-logical circuit 92 and theinverted pit string information data iDp from the data inversion circuit93 based upon the input of the control signal Pc from the systemcontroller 97 as later explained, and a driving circuit 95 for drivingthe light modulator 78 based upon output data of the selector 94. Thesignal processor 74 finally includes a clock generator 96 for supplyingclock signals to the arithmetic-logical circuit 92 and a systemcontroller 97 for controlling, above all, the servo controller 82, basedupon the clocks from the clock generator 96.

With the above-described laser cutting device, the glass substrate 71 isrotated by the motor 79, at the same time as the glass substrate 71 isslid as it is kept in rotation.

Specifically, the servo controller 82 controls the slide motor 81 sothat, while the motor 79 is controlled to cause rotation of the glasssubstrate 71 in accordance with the CAV system, the slide motor 81 iscontrolled so that the radial feed pitch is one-half the track pitch(0.8 μm).

The laser light source 75 comprises a He-Cd laser, for example, andcauses the outgoing light L to be incident on the light modulator 76.The light modulator 76 of, for example, the acousto-optical effect type,modulates the light intensity of the outgoing light L from the laserlight source L in accordance with output data from the selector 94, andFacilitates the intensity-modulated beam on the photoresist surface ofthe glass substrate 71 via the prism 77 and the objective lens 78. As aresult, the photoresist is sensitized in accordance with output data ofthe selector 94.

The wavelength of the outgoing light L from the laser light source 75and the numerical aperture NA of the objective lens 78 are set so thatthe spot diameter of the converged modulated light beam on thephotoresist surface becomes equal to approximately one-fourth of thetrack pitch. Consequently, the photoresist is sensitized to a width of0.4 to 0.5 μm.

The data formatting circuit 91 divides the source data Ds from thecomputer into sectors and segments and appends sector or segmentaddresses and error correction codes to the sectors and segments toproduce the recording data Dw which is supplied to the downstream sidearithmetic-logical circuit 92. The data formatting circuit 91 alsoroutes tracking or focusing servo bytes S_(B) to the arithmetic-logicalcircuit 92.

The arithmetic-logical circuit 92 appends the servo bytes S_(B) from thedata formatting circuit 91 to the leading end of the recording data Dwfrom the data formatting circuit 91, above all, to the leading end ofthe segment-based recording data Dw. The arithmetic-logical circuit 92then performs pre-set arithmetic-logical operations on the recordingdata Dw, based upon the output timing of the clock signals from theclock generator 96, for conversion to pit string information data Dp.

As explained previously in connection with the reproducing method, therecording data Dw has the logical data corresponding to that of theplayback data from the arithmetic-logical circuit 31 shown in FIG. 8.That is, the data string is logically "1" at the boundary between thepit P and the mirror surface M and is logically "0" otherwise. However,since data indicating whether or not the pit P is to be formed isrequired for actual laser cutting, the present arithmetic-logicalcircuit 92 converts the above-mentioned logic of the data string intothe logic which depends upon the presence or absence of the pit P. Inaddition, with the optical disc 1 of the present embodiment, since theservo mark Ms of the servo region Zs is dephased by one-half of theclock, logical conversion in the arithmetic-logical circuit 92 isperformed using the double frequency clock signal the period of which isone-half of the period of the clock signal Sc employed for reproduction.

In laser-cutting the photoresist by the above-described laser cuttingdevice, if it is assumed that the objective lens 78, for example, isslid from the radially inner most side towards the radially outer mostside of the glass substrate 71, as a result of the sliding movement ofthe glass substrate 71 caused by the slide motor 81, the first portionon which laser cutting is performed is a portion offset by one-fourth ofthe track pitch from the track center Tc of the inner most track towardsthe center of the glass substrate 71. That is, the inner peripheralportion of the track center Tc of the radially inner most track isprocessed with laser cutting.

Consequently, during the initial stage, the control signal Pc, that is asignal indicating selection of the pit string information data Dp fromthe arithmetic-logical circuit 92, is outputted from the systemcontroller 97 to the selector 94. Based upon the input of the controlsignal Pc from the system controller 97, the selector 94 selects the pitstring information Dp from the arithmetic-logical circuit 92 and routesthe data Dp to the driving circuit 95.

The driving circuit 95 drives the light modulator 76 based upon the pitsting information Dp from the selector 94. As a result, the pit stringcorresponding to the pit string information data Dp is laser-cut on theinner peripheral portion of the track center Tc of the inner most trackon the photoresist surface of the glass substrate 71.

After the glass substrate 71 is rotated one complete turn as the lasercutting is made on the radially inner most track as described above, theobjective lens 78 is then positioned on the outer peripheral portion ofthe track center Tc of the radially inner most track. At this time, thesystem controller 97 outputs the control signal Pc, that is the signalindicating selection of the inverted pit string information data iDpfrom the data inverting circuit 93, to the selector 94, which thenselects the inverted pit string data information iDp from the datainverting circuit 93, based upon the input of the control signal Pc fromthe controller 97, and routes the selected data iDp to the drivingcircuit 95.

The driving circuit 95 drives the light modulator 76 based upon theinverted pit string information data iDp from the selector 94. As aresult, the inverted pit string corresponding to the inverted pitinformation data iDp is laser-cut in the outer peripheral portion of thetrack center Tc of the radially inner most track on the photoresistsurface on the glass substrate 71.

The attributes of the control signal may be determined by a methodcomprising counting the number of FG pulses from the FG generator 80 inthe system controller 97 for detecting the number of revolutions of theglass substrate 71 and determining the signal indicating the selectionof the pit string information data Dp from the arithmetic-logicalcircuit 92 or the inverted pit string information data iDp from the datainverting circuit 93 as the attributes of the control signals Pc.

Subsequently, pit strings and inverted pit strings are laser-cut in asimilar manner on the inner and outer peripheral sides of the pre-settrack centers Tc, respectively.

After laser cutting on the photoresist by the above-mentioned lasercutting device, development and electro-casting operations are performedfor completing the stamper. Using such stamper, the optical discs may beproduced in large quantities.

With the above-described laser cutting device, the laser beam L ismodulated in intensity by the light modulator 76. If the lightconverging position of the objective lens 78 is on a pre-set side, forexample, on the inner peripheral side, of the track center Tc as aresult of the laser spot scanning the inner peripheral side of the trackcenter Tc, that is if the laser spot scans the inner peripheral side,the pit string information data Dp enters the driving circuit 95,driving the light modulator 76, via the selector 94, so that the laserbeam L from the laser light source 75 is modulated in intensitydepending on the pit string information data Dp. As a result, the pitstring corresponding to the pit string information data Dp is formed onthe inner peripheral side of the track center Tc.

If the light converging position of the objective lens 78 is on apre-set side, for example, on the outer peripheral side, of the trackcenter Tc as a result of the laser spot scanning the outer peripheralside of the track center Tc, that is if the laser spot scans the outerperipheral side, the inverted pit string information data iDp enters thedriving circuit 95, driving the light modulator 76, via the selector 94,so that the laser beam L from the laser light source 75 is modulated inintensity depending on the inverted pit string information data iDp. Asa result, the pit string corresponding to the inverted pit stringinformation data iDp is formed on the outer peripheral side of the trackcenter Tc.

Consequently, the completed optical disc 1 has the pit string comprisingthe pits P and the lands M and the inverted pit string comprising thepits P and the lands M inverted from those of the pit string, which areformed on the inner and outer peripheral sides of the track center Tc,respectively.

Since the pit string, comprising pits P and lands M, and the invertedpit string, which is the inversion of the pits P and the lands M of thepit string, are formed on the inner and outer peripheral sides of thetrack center Tc of the optical disc 1 of the present embodiment,respectively, if the pit string and the inverted pit string on the innerand outer peripheral sides of the track center Tc are thought of as onetrack of data, the proportion of the pits P and the lands M in the dataregion Zd per track in the pit string is equal to that in the invertedpit string.

Thus, when transcribing the recording pattern on the stamper, that isthe pit string pattern comprising the pits P and the lands M, onto theresin substrate by the injection molding method, for producing theoptical disc 1, it becomes possible to maintain uniform flowing velocityof the molten resin into the cavity, thereby eliminating molding defectsin the course of the preparation of the optical discs.

On the other hand, if the inner peripheral pit string and the outerperipheral inverted pit string are scanned with the sole beam spot BS,the beam spot is modulated (that is diffracted) by, for example, theinner peripheral pit P and subsequently modulated, that is diffracted,by the outer peripheral side pit P. That is, the upper half, for exampleof the beam spot BS is modulated by the inner peripheral pit P, whilethe lower half of the beam spot BS is modulated by the outer peripheralpit P.

As a result, if the reflected light Lr of the laser beam L isphoto-electrically converted by a photodetector 15 having its lightreceiving area divided into four equal parts, as shown in FIG. 11, thelight volume of the reflected light Lr in its entirety becomessubstantially equal. At this time, the reflected light Lr, modulated bythe inner peripheral side pits P, is incident on light-receiving regions15a and 15b associated with the inner peripheral side and therebyconverted into electrical signals. Subsequently, the reflected light Lr,modulated by the outer peripheral side pits P, is incident onlight-receiving regions 15c and 15d associated with the outer peripheralside and thereby converted into electrical signals.

Thus, if the push-pull signal Dpp is produced based upon the electricalsignals from the photodetector 15, and playback signals are recoveredbased upon the push-pull signal Dpp, an optimum dc balance may bemaintained, that is the digital sum value (DSV) may be reducedsatisfactorily to zero. Since the optimum dc balance may be achievedspontaneously without modulation for stabilizing the dc balance, such asby EFM, it becomes possible to record the recording data directly as thepit information, without the necessity of modulation, such as EFM,thereby enabling high-density recording of the recording data.

As another means for efficiently reducing the DSV to zero, a scramblercircuit 101, shown by a double-dotted chain line frame, may be insertedupstream of the data formatting circuit 91 of the laser cutting deviceshown in FIG. 17. The source data Ds from the computer is scrambled inaccordance with the scrambling rule registered in a memory built intothe scrambler circuit 91, and the resulting scrambled data is caused toenter the downstream side data formatting circuit 91. In this case, adescrambler circuit 102, indicated by a double dotted chain line framein FIG. 8, is inserted downstream of the arithmetic-logical unit 31. Thedescrambler circuit 102 has a memory therein in which a scrambling ruleopposite to that of the scrambler circuit 101 is stored. Output data ofthe descrambler circuit 102 is the same as the recording data Dwprepared by the data formatting circuit 91.

The servo marks Ms, making up the servo region Zs, are formed with phaseshift by one-half the clock, so that various data detected on the clockbasis may be demarcated from the servo marks in timing, with the resultthat the servo marks Ms, that is the servo region Zs, may be easilydetected by the unique pattern detection circuit 26 of the reproducingsystem for enabling high-speed accessing of the recording data.

With the reproducing method of the present embodiment, the laser beam Lfor reproduction is radiated on the track center Tc, and the push-pullsignal Dpp is produced by calculation based upon the first and secondpush-pull signals S₁ and S₂ produced by light diffraction by the innerand outer peripheral pits P. The kinds of the recording pattern of theoptical disc 1 may be detected based upon the polarity of the push-pullsignal Dpp and the signal level of the RF signal S_(RF). That is, theoptical disc 1 has such recording pattern comprising the data region Zdcarrying actual recording data and the mirror areas Zm and Zs employedfor focusing servo control and tracking servo control, respectively.

Since the pit string constitution of the data region Zd comprises a pitstring made up of pits P and the lands M on the inner peripheral side ofthe track center Tc, and an inverted pit string, made up of the pits andthe lands inverted in array from those of the pit string, the playbacklaser beam L is necessarily modulated by one of the pits P.Consequently, the RF signal S_(RF), obtained after conversion of thereflected light Lr into electrical signals, is of a mid signal level.

The mirror area Zm is constituted solely by the lands M, so that thelight volume of the light Lr reflected thereby is abundant, such thatthe RF signal S_(RF) after conversion into electrical signals is of ahigh level. The servo region Zs, above all, its region carrying theclock marks Mc, is set as usual so that the light volume of the lightreflected thereby becomes least, so that the signal level of the RFsignal after conversion of the reflected light Lr into electricalsignals is of the low signal level.

In short, whether the recording pattern to be reproduced is that of thedata region Zd, mirror area Zm or of the servo region Zs, above all,that of the clock marks Mc, can be easily discerned based upon thesignal level of the RF signal produced on converting the reflected lightLr to the electrical signals.

If the recording pattern discerned as described above is that of thedata region Zd, the information signals recorded in the data region Zdare reproduced based upon the push-pull signal Dpp calculated based uponthe first and second output signals S₁ and S₂.

In this manner, the information signals recorded in the data region Zdof the optical disc 1 having the pit string of the pits P and the landsM on the inner periphery of the track center Tc and the inverted pitstring of the pits P and the lands M inverted in array from those of thepit string are reproduced based upon the push-pull signals Dpp arereproduced. Consequently, the dc balance may be optimized, that is thedigital sum value (DSV) may be reduced to zero spontaneously without thenecessity of modulation for stabilizing the Dc balance, such as EFM.Since the recording data may be directly recorded as the pit informationwithout modulation of the recording data, such as EFM, with consequentincrease in data length, it becomes possible to realize a high recordingdata density.

Above all, the RF signal D_(RF) is detected by detection by partialresponse PR (1, 1) and when the detection by PR (1, 1) is achieved byViterbi decoding, it becomes possible to eliminate inter-symbolinterference at the time of signal reproduction accompanying highdensity recording and to improve the S/N ratio of the playback signal.

The data region Zd of the above-described recording pattern may bediscriminated based upon discrimination of the mid signal level of theRF signal SRF and the signal level of the push-pull signal Dpp not beingzero, thereby enabling high speed accessing of the recording data.

In the present embodiment, the optical disc 1 has at least one clockmark Mc in the servo region Zs, that is a region in which the RF signalS_(RF) is of a low level and the push-pull signal Dpp is zero, and theclock signal Sc is generated based upon detection of the clock mark Mcduring reproduction of the optical disc 1, in order to render itpossible to generate stable clock signals Sc and to reproduce therecording data satisfactorily.

On the other hand, the tracking error signal is generated by radiatingthe laser beam L on the optical disc 1, sampling RF signals D_(RF)corresponding to the light volume of the reflected light Lr at servo pitpositions for producing three-phase signals, finding the differences ofthe three-phase signals and by periodically switching and selecting thedifference signals. In this manner, the beam spot BS may be capturedquickly, accurately and stably with respect to the track center Tcduring signal reproduction or track jump, thereby enabling reduction inthe time since seek start until reproduction and enabling high-speedaccessing of the recording data.

The optical disc 1 according to the second embodiment is explained byreferring to FIG. 18, in which parts or components similar to those ofFIG. 4 are correspondingly numbered and the description for these partsor components is not made for simplicity.

With the optical disc 1 of the present second embodiment, the mirrorarea Zm separating the servo area Zss in the servo region Zs from thedata region Zd is alternately arrayed at the leading end or at thetrailing end of the servo area Zs for every other track.

In this manner, the mirror area Zm is not consecutive in the radialdirection of the optical disc 1, such that there is no danger of themolten resin flowing rapidly through the mirror area Zm towards theouter rim of the cavity during preparation of the optical disc 1. Theresult is that there is no risk of occurrence of "ghosts", that isfractured edge portions of the pit P in the servo region Zs of thecompleted optical disc 1.

In the previously described first embodiment, the mirror areas Zm areprovided at the leading and trailing ends of the servo regions Zs, andtwo adjoining regions are distinguished by shifting the clock timing inthese regions. In the present embodiment, there are pits formed in themirror area Zm. However, since the data region Zd is not separated fromthe servo area Zss by a distance equal to an integer number of clocksand there is formed a mirror area in one of the pit strings, the dataregion may be distinguished from the servo area.

Referring to FIG. 19, a third embodiment of the optical disc 1 isexplained. Parts and components similar to those shown in FIG. 4 arecorrespondingly numbered and description of those parts or components isnot made herein for simplicity.

The optical disc 1 of the third embodiment is similar to that of thefirst embodiment shown in FIG. 4, with the exception that polarity marksMp1 and Mp2, each having a pit length equal to two clocks, are formedastride the boundary of the data region Zd and the mirror area Zmseparating the servo area Zss in the servo region Zs from the dataregion Zd, and that the array pattern as described below is used as thearraying pattern of servo pits P making up the servo area Zss.

That is, the same arraying pattern of the servo pits P making up theservo area Zss is repeated at intervals of three tracks. Specifically,if a domain having a length of two clocks from the inchoate end of theservo area Zss is a region A, a domain having a length of two clocksfrom a position five clocks apart from the inchoate end of the region Ais a region B and a domain having a length of two clocks from a positionfive clocks apart from the inchoate end of the region B is a region C,there are three patterns of pit array combinations included in each ofthese regions. Meanwhile, the regions A to C are separated from oneanother by mirror surface sections or lands M.

That is, in the illustrated embodiment, the inner and outer peripheralregions of the first track T₁ include pits P in the regions A and B andpits P in the regions A and C, respectively. In other words, the pits Pare present in both the inner and the outer peripheral sides and eachone pit P exists in the inner peripheral side in the B region and in theouter peripheral side in the region C. In the first track T₁, the pits Pin the region A and the pits P in the regions B and C are used as clockmarks Mc for clock detection and as servo marks Ms for tracking errordetection, respectively.

Next, the inner and outer peripheral regions of the second track T₂include pits P in the regions B and C and pits P in the regions A and B,respectively. In other words, the pits P are present in both the innerand the outer peripheral sides and each one pit P exists in the outerperipheral side in the region A and in the inner peripheral side in theregion C. In the second track T₂, the pits P in the region B and thepits P in the regions A and C are used as clock marks Mc for clockdetection and as servo marks Ms for tracking error detection,respectively.

Finally, the inner and outer peripheral regions of the third track T₃include pits P in the regions A and C and pits P in the regions B and C,respectively. In other words, the pits P are present in both the innerand the outer peripheral sides in the region C and each one pit P existsin the inner peripheral side in the region A and in the outer peripheralside in the region B. In the third track T₂, the pits P in the region Cand the pits P in the regions A and B are used as clock marks Mc forclock detection and as servo marks Ms for tracking error detection,respectively.

Of the polarity marks Mp, the leading polarity mark Mp1 is made up oftwo consecutive clocks, namely one clock at the trailing end of the dataregion Zd and one clock in the mirror area Zm, these two clocks lyingastride the data region Zd and the mirror area Zm. On the other hand,the trailing polarity mark Mp2 is made up of two consecutive clocks,namely one clock at the leading end of the data region Zd and one clockin the mirror area Zm, these two clocks lying astride the boundary tothe data region Zd and the mirror area Zm.

These leading and trailing polarity marks MP1 and MP2 are both formed onthe outer peripheral side. When reproduced as the push-pull signal Dppby the reproducing system shown in FIG. 8, the polarity marks arereproduced as the push-pulls signal having the (+) polarity. That is,the servo region Zs or the servo area Zss in the present thirdembodiment, when viewed from the push-pull signal Dpp, is equivalent tobeing sandwiched between the (+) polarity push-pull signals. As aresult, the servo region Zs, and in particular the clock mark Mc can bedetected easily by the pattern detection circuit 27, thereby expeditingdata accessing.

Referring to FIG. 20, a fourth embodiment of the optical disc 1 isexplained. Parts and components similar to those shown in FIG. 19 arecorrespondingly numbered and description of those parts or components isnot made herein for simplicity.

The optical disc 1 of the fourth embodiment is similar to that of thethird embodiment shown in FIG. 19, with the exception that the followingarraying pattern is used for the pits constituting the servo area Zss(servo pits).

That is, the same arraying pattern of the servo pits P making up theservo area Zss is repeated at intervals of three tracks. Specifically,if a domain having a length of two clocks from the inchoate end of theservo area Zss is a region A, a domain having a length of two clocksfrom a position six clocks apart from the inchoate end of the region Ais a region B and a domain having a length of two clocks from a positionsix clocks apart from the inchoate end of the region B is a region C,there are three patterns of pit array combinations included in each ofthese regions.

That is, in the illustrated embodiment, pits P are formed in the innerperipheral side of the first track T₁, while pits P are formed in theouter peripheral side thereof, as from the trailing end of the leadingpolarity mark Mp1 as far as the region A, as traversing the mirror areaZm along the track direction. In addition, pits P are formed as from theregion C up to the leading end of the Region A, as traversing the mirrorarea Zm along the track direction. In short, the pits P are present inboth the inner and the outer peripheral sides and one pit P exists inthe inner peripheral side in the B region and in the outer peripheralside and another pit P exists in the region C. In the first track T₁,the pits P in the region A and the pits P in the regions B and C areused as clock marks Mc for clock detection and as servo marks Ms fortracking error detection, respectively.

In the inner and outer peripheral sides of the second track T₂, thereare formed pits from the region B across the region C and from theregion A through the region B, respectively. In short, the pits P arepresent in both the inner and the outer peripheral sides of the region Band one pit P exists in the outer peripheral side in the A region andanother pit P exists in the inner peripheral side in the region C. Inthe second track T₂, the pits P in the region B and the pits P in theregions A and C are used as clock marks Mc for clock detection and asservo marks Ms for tracking error detection, respectively.

In the inner peripheral side of the third track T₃, there are formedpits P from a position corresponding to the trailing end of the leachingpolarity mark Mp1 through the region A and from the region C to aposition corresponding to the leading end of the trailing end polaritymark Mp2, while a pit P is formed from the region C through the regionC. In short, the pits P are present in both the inner and the outerperipheral sides of the region C and one pit P exists in the innerperipheral side in the A region and another pit P exists in the outerperipheral side in the region B. In the third track T₃, the pits P inthe region C and the pits P in the regions A and B are used as clockmarks Mc for clock detection and as servo marks Ms for tracking errordetection, respectively.

Turning to the servo region Zs of the optical disc 1 according to thepresent fourth embodiment, a region thereof presenting the lands in boththe inner and outer peripheral sides of a given track, such as thesecond track T₂, indicated as (M) in FIG. 20, is spaced apart from theclock mark Mc by a distance as measured from the trailing end of theregion (M) as far as the leading end of the clock mark Mc, equal to sixclocks. Besides, the region (M) of a given track is spaced apart fromthe region (M) of adjacent tracks by a distance as measured between thetrailing end of the land of such given track, such as the second trackT₂, and the leading end of the land of an adjacent track, such as thethird track T₃, equal to two tracks.

In the optical disc of the fourth embodiment, since the leading andtrailing end polarity marks are formed in the same manner as in theoptical disc of the third embodiment, the servo region, in particularthe clock marks, can be detected easily by the unique pattern detectioncircuit, thereby enabling expedited data accessing.

In distinction from the third embodiment, pits P are continuously formedin the first track T1 and in the third track T3 for traversing the landalong the track direction. Besides, in the third embodiment, since pitsare formed for traversing the lands present between the regions A and Band between the regions B and C, there is no possibility for the mirrorarea Zm and the mirror surface to be formed continuously along theradius of the optical disc 1 and hence there is no possibility for themolten resin to flow quickly through the mirror area Zm and through themirror surface towards the outer rim of the cavity during fabrication ofthe optical disc 1. The result is that there is no risk of occurrence ofso-called ghosts, that is fractured edges of the pits P in the servoregion Zs of the completed optical disc 1.

Although the present invention is applied in the above-describedembodiments to the replay-only optical disc 1, the present invention mayalso be applied to a magneto-optical disc in which the pit stringinformation is formed on the segment basis and the information is readout by reproduction under the Kerr effect.

What is claimed is:
 1. A reproducing apparatus for reproducing aninformation signal recorded on an optical recording medium having afirst pit string having a succession of pits and mirror surfacesections, the first pit string being formed on one side of a trackcenter as a reference, and a second pit string formed on an oppositeside of the track center and having pits and mirror sections in an arraywhich is the logical inverse of the pits and the mirror surface sectionsof the first pit string, wherein a laser beam radiated onto the trackcenter is used to access information signals represented by the pits andthe mirror surface sections, said reproducing apparatus comprising:anoptical pickup for radiating a laser beam having a spot diameter astridesaid first and second pit strings onto said track center and outputtinga detection signal based on the volume of the reflected light, means forcalculating a push-pull signal based upon the detection signal from saidoptical pickup, and reproducing means for reproducing said informationsignal based upon polarity inversion of said push-pull signal from saidcalculating means.
 2. A reproducing apparatus for reproducing aninformation signal recorded on an optical recording medium having asampled servo system recording format having a data region for recordinginformation signals and a servo region having a servo area for recordingservo data, in which the data are accessed depending on pre-set clocksignals, wherein a first pit string and a second pit string each formedby a succession of pits and mirror surface sections are formed on oneand the other sides of a track center, respectively, as a reference, sothat a laser beam radiated onto the track center is used to accessinformation signals represented by the pits and the mirror surfacesections, and wherein only in the data region are the pits and mirrorsurface sections of the first string the logical inverse of the pits andmirror surface sections of the second pit strings, said reproducingapparatus comprising:an optical pickup for radiating a laser beam havinga spot diameter astride said first and second pit strings onto saidtrack center and outputting a detection signal based on the volume ofthe reflected light, means for calculating a push-pull signal based uponthe detection signal from said optical pickup, and reproducing means forreproducing said information signal recorded on said data region basedupon polarity inversion of said push-pull signal from said calculatingmeans.
 3. The reproducing apparatus as claimed in claim 2 wherein:saidservo area and the data region of the optical recording medium areseparated from each other by a mirror area formed in the servo area onlyby said mirror surface section, and wherein said reproducing meansseparates the signal reproduced from the servo area and the signalreproduced from the data region by detecting said mirror area.
 4. Thereproducing apparatus as claimed in claim 2 wherein:the pits formed insaid servo area of the optical recording medium are dephased by beingoffset by one-half the clock pitch with respect to the pits formed insaid data region, and wherein said reproducing means separates thesignal reproduced from the servo area and the signal reproduced from thedata region by detecting said dephasing.
 5. The reproducing apparatus asclaimed in claim 2 wherein:both the first pit string and the second pitstring are formed in the servo area, and the servo area includes a clockmark for detecting the clock signals and a servo mark for producing thetracking information, one of the first and second pit strings of theservo mark being formed as pits and the other of the first and secondpit strings being formed as a mirror surface section, and wherein saidreproducing means generate the clock signals based upon the results ofdetection of said clock mark.
 6. The reproducing apparatus as claimed inclaim 3 wherein:said reproducing means determine, based upon thepolarity of said push-pull signal and the signal level of said detectionsignal, from which of the servo area, mirror area and the data regionthe reproduced signals have been derived.
 7. The reproducing apparatusas claimed in claim 3 wherein:both the first pit string and the secondpit string are formed in the servo area, and the servo area includes aclock mark for detecting the clock signals and a servo mark forproducing the tracking information, one of the first and second pitstrings of the servo mark being formed as pits and the other of thefirst and second pit strings being formed as a mirror surface section,and wherein said reproducing means generate the clock signals based uponthe results of detection of said clock mark.
 8. The reproducingapparatus as claimed in claim 7 wherein:said reproducing meansdetermine, based upon the polarity of said push-pull signal and thesignal level of said detection signal, from which of the servo area,mirror area and the data region the reproduced signals have beenderived.
 9. The reproducing apparatus as claimed in claim 8 wherein:saidreproducing means determine the signal level of the detection signalbased upon a three-level logic of high, mid and low levels.